The referenced U.S. Pat. No. 4,438,372, Zuchtriegel, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, describes a circuit to operate one or more fluorescent lamps at a frequency which is above network or power frequency. The received power frequency, at 50 or 60 Hz, depending on national standard, is rectified, and then converted to a much higher frequency. It has been found that fluorescent lamps operate more efficiently at frequencies substantially higher than network power frequency.
The circuit, as described in the referenced literature and patents, utilizes, hence, a power network rectifier to which a push-pull frequency of two alternatingly switching transistors is connected. A control circuit is provided to control the transistors for alternate switching. A center junction between the transistors forms one terminal of the supply circuit to the fluorescent lamp or lamps. Each one of the fluorescent lamps has a series resonance circuit associated therewith, which includes a resonance inductance, a coupling capacitor and a resonance capacitor. The supply lines for the fluorescent lamps are connected to a first electrode via the coupling capacitors and the resonance inductances to connect to the center junction between the two transistors. A further line from the second electrode of the lamps is connected to the negative terminal of the power network rectifier. To remove operating power from the lamps, for example if one of the lamps should burn out or is to be replaced, a turn-off circuit is provided which includes a diode in series with a resistor and a thyristor with a trigger circuit which connects the base of the transistor connected to the positive terminal of the power rectifier with the negative terminal of the power rectifier, through a resistor which, in turn, is connected to the positive terminal of the power rectifier and to the center junction between the diode and the other resistor. A reenergization circuit is likewise provided.
The basic circuit, without the turn-off and the reenergization circuit, is described in the Siemens literature, for example in the book "Elektronischaltungen" ("Electronic Circuits") by Walter Hirschmann, chapter B3.12, pp. 147-151, "Elektronisches Vorschaltgerat fur neue Leuchtstofflampen" ("Electronic Ballast Circuit for New Fluorescent Lamps") 50 W/220 V. This detailed description is also reflected in the English-language literature without, however, all the theoretical considerations, see Siemens publication "SIPMOS Transistors, SIEMENS Application Notes 1983".
The connection of the fluorescent lamp or lamps can be placed, selectively, between the positive terminal of the power rectifier and the center junction between the serially connected switching transistors, or between the center junction of the two transistors and the negative terminal of the power rectifier. It has been found preferable to connect the fluorescent lamps in parallel with the center junction between the two transistors and the negative terminal of the power rectifier, as will be described in detail below.
U.S. Pat. No. 4,438,372, Zuchtriegel, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, describes a similar ballast or energizing circuit having a self-starting push-pull switching arrangement. The network circuit, including the push-pull switch, further includes a disconnect circuit, as described above. In case a lamp is defective, the push-pull switching circuit is disabled by firing of the thyristor. After the lamp, which may be a single lamp or one of a plurality of lamps, has been replaced, the push-pull circuit will not start on its own. It is necessary first to remove the holding or firing current from the thyristor, so that the thyristor can revert to blocking state. Consequently, if for example one lamp of a plurality of lamps has to be disconnected or replaced, it is necessary to first switch off the circuit for all the lamps in which it is used, before the circuit can be restarted, by starting the push-pull oscillator again to generate oscillations.
U.S. Pat. No. 4,481,460 describes an inverter with a disconnect circuit which is particularly designed for use with discharge lamps with heatable electrodes. A thyristor is used as a switching element for a disconnect or turn-off circuit connected in the heating circuit of the lamp. The thyristor connects the negative terminal of the power rectifier with the lamp heater circuit. The thyristor receives a holding current from the electrode closest to the charge capacitor. A disconnect winding of the saturation transformer is coupled in parallel with the thyristor via diodes and the starting capacitor is coupled in parallel with the thyristor via a resistor. In this disconnect circuit, it is not necessary to disconnect the alternating current power supply after exchange of a lamp in order to permit the inverter to start again. When a defective lamp is removed, the thyristor also loses its holding current, so that it will revert to blocking state.